This invention relates generally to diagnostic equipment for vehicle engines, and more particularly to portable and convenient oxygen sensing equipment for verifying that the on-board oxygen sensor is operating correctly, and in accordance with emissions control regulations.
Most computer controlled gasoline vehicle engines are equipped such that the air-fuel ratio is controlled by a 0–1 V oxygen sensor as specified, for example, in U.S. Pat. No. 3,745,768. Such oxygen sensors are required to operate from a maximum of 175 mV (lean) to a minimum 800 mV (rich) and should be able to move between these voltage limits, from lean to rich, within 100 mS as detailed in emissions regulations, such as those set forth in the California Bureau of Automotive Repairs Oxygen Sensor Test. The ideal stoichiometric condition of 14.7 to 1 is approximately 500 mV, which the on-board control computer tries to maintain by cutting back on fuel when the oxygen sensor reads rich and increasing fuel when the oxygen sensor reads lean. The oxygen sensor is the primary feedback sensor controlling emissions and fuel mileage and it is important that it is operating correctly. Late model cars with OBD2 (On-Board Diagnostics 2) emissions control equipment (required by the Federal Government since 1996) may be able to identify slow or faulty sensors, but pre-1996 automobile on-board diagnostic systems (OBD1 systems) only have the capability to detect catastrophic failures.
To test this on-board oxygen sensor, to ensure operation within emissions control regulations, the California Bureau of Automotive Repairs Oxygen Sensor-Test, for example, requires that propane be fed into the air intake to enrich the engine, so that when the flow of propane is turned off, the engine will go lean and the oxygen sensor signal will fall below 175 mV. When this is observed on an expensive digital storage oscilloscope (DSO), the throttle is snapped, forcing the engine to go rich very quickly. The oxygen sensor signal must exceed 800 mV within 100 mS of moving above 175 mV, as measured on the oscilloscope using 100 mS/Div time base, or it is considered slow or faulty, and should be replaced. With some 4-wire oxygen sensing systems, which can cost in excess of $300, an accurate test is essential, but this official test, although it is required for smog technicians, is time consuming because of the propane usage requirement, and accurate to only about 20%, when trying to read 100 mS on a single division. As a result, most oxygen sensors are replaced on a whim, a guess, or according to arbitrary vehicle mileage levels. This is a very expensive and unnecessary approach for the consumer, particularly if this arbitrary course of action does not resolve the actual emissions system problem. Many technicians try to diagnose the condition of an oxygen sensor with a scanner, such as that disclosed in U.S. Pat. No. 4,831,560, but since these use the ALDL serial data diagnostic link, which comprises an under-dash serial data port required as standard equipment in automobiles under U.S. regulations, with perhaps 500 mS update times, they can only capture a voltage on a random basis, and cannot identify a bad oxygen sensor, unless its failure is catastrophic. Also, they cannot show real time operation of the oxygen sensor, where other engine faults can be seen, especially while the vehicle is being operated, and do not have any capability of simulating to the computer to confirm correct operation.